Structures of a vehicle, for example, members requiring strength, such as various pillars and reinforcements are manufactured through heating. Heating is classified into indirect heating and direct heating. An example of the indirect heating is so-called furnace heating of inputting a workpiece into a furnace and heating the workpiece through control of the temperature of the furnace. On the other hand, examples of the direct heating include induction heating of heating a workpiece by supplying the workpiece with an eddy current and direct resistance heating of heating a workpiece by directly supplying the workpiece with a current.
A so-called tailored blank material in which characteristics are partially changed by joining different types of steel plates is used as a component of a vehicle body. For example, JP 2004-58082 A discloses a method of butt-welding ends of members having different materials or different thicknesses to each other and then performing press working.
However, as for the tailored blank material, it is necessary to butt-weld plural materials. The number of working processes increases and thus the tailored blank material is not suitable for mass production.